GPRS is a standard on packet data in GSM systems developed to meet the increased demand for wireless data services. It enables efficient transfer of user data packets between mobile terminals and external packet data networks, such as the Internet. GPRS results in a number of advantages, in particular with regard to resource utilization, billing, transfer rates and access times.
In order to integrate GPRS into an existing GSM network, two new types of core network nodes have to be introduced. These are the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). GGSN acts as an interface towards external packet data networks, while SGSN is responsible for packet delivery to and from terminals within a certain area.
The capabilities of GPRS networks are defined through Network Operating Modes (NOM). In NOM-I networks, an interface is configured between the Mobile Switching Center (MSC) and SGSN, enabling the nodes to interact. This so-called Gs interface is a prerequisite for NOM-I. Neither NOM-II, which is the default mode today, nor NOM-III has the optional Gs-interface configuration. By means of the Gs interface, enhanced efficiency and co-ordination of GPRS and non-GPRS services can be achieved.
In NOM-I, GPRS capable terminals perform combined procedures, such as combined GPRS and International Mobile Subscriber Identity (IMSI) attach and combined location area/routing area (LA/RA) update, towards the SGSN. A configured Gs interface is actually not in active use until the terminals have received a NOM-I indication and begin performing these combined procedures. Circuit Switched (CS) paging for terminals that are both GPRS and IMSI attached is then sent over the Gs interface because of the advantages obtained thereby.
A poor or non-working Gs interface leads to serious signaling and transfer problems. If the Gs interface is not fully reliable, pages that are normally routed over this interface are likely to get lost. CS pages sent via the Gs interface to terminals both GPRS and IMSI attached will for instance not reach the terminals if this interface is down. This considerably reduces the value of the service in the eyes of an end user.
A further problem associated with Gs interface failure is that combined procedures cannot be performed. The terminals will repeatedly try to complete the procedures, inhibiting CS calls for a relatively long period of time. The effect gets particularly serious when the end user repeatedly turns the phone on and off in belief that the phone has crashed, since the try period starts over every time the terminal is turned on.
Accordingly, the Gs interface handling of conventional telecommunication systems is far from satisfactory and there is a considerable need for an improved signaling method.